Fighters from the Islamic State stormed into eastern Syria from their Iraq strongholds Thursday, overrunning or co-opting Syrian tribes and rebel groups holding the strategic towns along the border that control both the main highways and the bulk of Syria’s oil production.

Fighters from the Islamic State, which declared itself a new caliphate Sunday, overcame the last bastions of al Qaida’s Nusra Front in both Deir el Zour, the capital of the province by the same name, and Abu Kamal.

The startling advance was the first major sign that the Islamic State’s spectacular military success in Iraq had won it new sympathizers among Sunni Muslims in Syria.

Most of the militant groups battling the regime of President Bashar al Assad had rejected the Islamic State’s claim that it had established a caliphate in the lands it controls, led by Abu Bakr al Baghdadi, the head of what formerly was known as the Islamic State of Iraq and Syria. But Nusra, which has fought a bitter battle with Baghdadi’s troops over tactics and strategy in Syria, has remained silent on the legitimacy of the caliphate announcement.

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On Thursday, the Syrian Observatory for Human Rights said that Nusra had withdrawn its units from Deir el Zour, the province’s oil fields and the refineries, while powerful Sunni tribes in the area that previously had supported Nusra quickly endorsed the new caliphate.

Unconfirmed but credible reports also appeared in local media accounts that multiple units previously aligned or directly controlled by Nusra also had defected and sworn allegiance to the Islamic State and its new caliphate.

The tribes in the towns of Mayadeen, al Ashara and Burqas, all long time allies of Nusra and the long rumored birthplace of Nusra commander Abu Mohammed al Golani, openly swore allegiance to the Islamic State, according to videos released on the Internet.

“The clans of the city of Ishara, and the villages around it . . . and all of the factions in these areas . . . announce before God that they will cease fighting with the Islamic State,” a tribal leader said in one video. The man, described as a tribal leader, also referred to Baghdadi as the “Caliph Ibrahim.”

The oil fields around Deir Azzour represent a possible output of 75,000 barrels of oil production per day when properly functioning, a large financial windfall for the new caliphate, which has seen its financial and military resources grow exponentially since it took over Mosul, Iraq’s second largest city, on June 10, then swarmed through much of the Sunni Muslim areas of central and northern iraq.

The Islamic State captured hundreds of millions of dollars worth of military equipment in its advance and seized additional millions when it took over the Iraqi central bank in Mosul, one of three branches.

Rami Abdurrahamn, the Syrian Observatory’s director, reported on Facebook that some Nusra units remained engaged in combat in the area but appeared to be on the brink of collapse due to defections and withdrawals.

Nusra’s future in light of the rise of their bitter rivals in the Islamic State remains very much in question and analysts have noted that one key Twitter account often used to disseminate information from the group has remained silent since Mosul fell to the Islamic State.